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updated zipmap documentation to match the implementation

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1 parent 8c67007 commit bfded2aa511f70bb07941698f883b38de68c7773 @pietern pietern committed Apr 1, 2010
Showing with 9 additions and 15 deletions.
  1. +9 −15 zipmap.c
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@@ -42,10 +42,11 @@
/* Memory layout of a zipmap, for the map "foo" => "bar", "hello" => "world":
*
- * <status><len>"foo"<len><free>"bar"<len>"hello"<len><free>"world"
+ * <zmlen><len>"foo"<len><free>"bar"<len>"hello"<len><free>"world"
*
- * <status> is 1 byte status. Currently only 1 bit is used: if the least
- * significant bit is set, it means the zipmap needs to be defragmented.
+ * <zmlen> is 1 byte length that holds the current size of the zipmap.
+ * When the zipmap length is greater than or equal to 254, this value
+ * is not used and the zipmap needs to be traversed to find out the length.
*
* <len> is the length of the following string (key or value).
* <len> lengths are encoded in a single value or in a 5 bytes value.
@@ -62,22 +63,15 @@
* or even in order to add a key/value pair if it fits.
*
* <free> is always an unsigned 8 bit number, because if after an
- * update operation there are more than a few free bytes, they'll be converted
- * into empty space prefixed by the special value 254.
+ * update operation there are more than a few free bytes, the zipmap will be
+ * reallocated to make sure it is as small as possible.
*
* The most compact representation of the above two elements hash is actually:
*
- * "\x00\x03foo\x03\x00bar\x05hello\x05\x00world\xff"
+ * "\x02\x03foo\x03\x00bar\x05hello\x05\x00world\xff"
*
- * Empty space is marked using a 254 bytes + a <len> (coded as already
- * specified). The length includes the 254 bytes in the count and the
- * space taken by the <len> field. So for instance removing the "foo" key
- * from the zipmap above will lead to the following representation:
- *
- * "\x00\xfd\x10........\x05hello\x05\x00world\xff"
- *
- * Note that because empty space, keys, values, are all prefixed length
- * "objects", the lookup will take O(N) where N is the numeber of elements
+ * Note that because keys and values are prefixed length "objects",
+ * the lookup will take O(N) where N is the number of elements
* in the zipmap and *not* the number of bytes needed to represent the zipmap.
* This lowers the constant times considerably.
*/

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